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This is pretty cool....
To mimic Gmail’s threads we need a way for Outlook to group by subject, but ignore the FW: and RE:. Luckily, it has just such a feature. It’s called Conversation, and it’s an optional column just like date received and subject. In your Inbox, go to the pane that lists your emails. Right-click any of the field headings, like From, or Subject, and select Field Chooser. You’ll get a list of additional fields that pertain to your emails. Click and drag Conversation to the other field headings. You’ll notice it looks just like subject, but ignores FW: and RE:.